This invention generally relates to tracking apparatus and to methods for tracking articles. More specifically, the present inventions relates to such apparatus and methods that are well suited for use in an optical coordinate measuring machine.
Optical coordinate measuring machines are used to measure large objects with high accuracy. The need for this type of measurement is found in many industries such as automotive, machinery, construction, robotics and aerospace.
In an optical coordinate measuring machine, a hand held probe is traversed along an object being measured. A series of transceivers are located in fixed positions around that object, and each transceiver, using an amplitude modulated laser beam, measures the distance between the transceiver and the probe. The distances measured by the various transceiver are processed to determine the location of the probe as it moves around the object being measured, and, from this, the precise contour of that object.
Typically, these transceivers include systems for tracking the probe so that the transceivers are able to aim their laser beams directly at the probe. In many optical coordinate measuring machines, an extremely high degree of precision--on the order of micrometers--is desired. In order to achieve this high degree of precision, it is important that the transceivers--or, more specifically, the tracking systems thereof--track movement of the probe with a correspondingly high degree of accuracy. The tracking system needs to be able to locate the retroreflector (article) quickly anywhere in a large volume. The tracking system is designed to lock a light beam onto a corresponding retroreflector to a high degree of precision so that offset errors are minimized. Minimizing the offset errors is important so that when one goes to measure the actual distances to the probes using the tellurometer principles, one gets the most precise measurement. The tracking system needs to be able to distinguish a real image of the beacon form a "ghost", reflected, spurious, or background image.